1. Field of the Invention
The present invention relates to a flow monitoring device of the type including a heat sensitive sensor/transducer which forms an active part of an electronic circuit. The circuit is capable of providing a measurable electrical output related to the rate at which temperature changes at the sensor/transducer take place for analyzing periodic variations of the temperature of the sensor/transducer as a result of pulsating flow stream through a passage near the sensor/transducer. That rate of change is typically a direct measurement of the velocity of the flow stream and the electronic circuit is operable to regulate the flow rate above or below a predetermined flow rate datum for a proscribed time period.
2. Description of the Prior Art
The use of thermistors as flow sensing transducers has been proposed, particularly for controlling the flow rates of liquids such as biomedical fluids. For example, U.S. Pat. No. 3,871,229 discloses a drip flow regulator using a thermistor as the transducer for sensing flow in an intravenous system. The transducer probe is positioned in an open chamber in line with the drop path for the intravenous fluid. The transducer consists of a thermistor insulated by a glass tube. That physical construction is needlessly complicated as it requires insulated wiring to and from the transducer as well as sealing for the wires at the entrance and exit from the open chamber. The temperature of impinging drops causes thermal exchange between each drop and the transducer resulting in an output which manifests a change of resistance in the circuit connected to the thermistor. Typically, such thermistor has a characteristic where the current increases linearly with voltage until self heating occurs whereby a drop in resistance permits continued current increase without further increase in voltage. It is this characteristic called the "negative volt-ampere" which is sometimes taken advantage of to provide an amplifiable signal which is compared with a standard reference datum for determining the drip rate and for maintaining the drip rate with a desired range.
U.S. Pat. No. 3,316,902 discloses a thermistor which is positioned in the nostril of an infant to monitor respiration. The flow of air directed past the thermistor provides a signal useful in measuring rate and continuity for purposes of signaling an alarm in connection with changes in or lack of respiration.
Typical of the use of thermistors for monitoring the flow of constant temperature fluid is the system disclosed in U.S. Pat. No. 3,199,348 in which a change of thermistor impedance occurs as a result of varying temperature of the flowing fluid. Another thermistor disclosed in U.S. Pat. No. 3,075,515 is used as a leg in a Wheatstone bridge circuit forming part of a blood flow measuring system.
In the system disclosed in U.S. Pat. No. 3,326,040, two thermistors are used to measure changes in the ambient temperature as well as changes in the flow in fluid temperatures whereby ambient conditions can be taken into account to improve the sensitivity of the thermistor used for measurement of flow.
In Bulletin 600 published by CGS/DATAMETRICS, a division of CGS Scientific Corporation of Watertown, Mass. entitled "Heat Sensor Finds Wide Applications in Fluid Flow Measurements" there is disclosed a heated sensor that is part of a bridge circuit that furnishes electric heating power to the sensor to maintain the temperature of the sensor above the ambient temperature upstream of the liquid flow.
Similarly, a flow meter operating on a temperature difference between sensors that are situated in legs of a bridge circuit is disclosed in German printed patent application number 27 53 118.
Other systems for monitoring a flowing liquid are disclosed in U.S. Pat. Nos. 3,279,251; 3,535,927; and 3,898,637.
While the foregoing patents and publications disclose a variety of uses of thermistors to measure fluid flows of gases or liquids none are specifically adapted to monitoring pulsatile flow nor is there any appreciation of the importance of controlling the transducer to react at a specific time relative to the periodicity of the flowing fluid. These requirements are critical to the measurement of flow of fluid in a drug delivery system.